Consulting-Specifying Engineer - December 2007 - (Page S23) High Headroom Low headroom can spell death for a surge protection device; knowing where to set headroom is the first step in protecting an SPD investment. BY ROGER EDELSON, MSEE, Power Partners Inc., Berkeley, Calif. and DION NERI, EE, MCG Surge Protection, Deer Park, N.Y. or How to Prevent Premature SPD Death N 2 N 1 L ow headroom can be uncomfortable in a vehicle, but in a surge protection device (SPD), it can result in the destruction of the protector. Knowing where to set headroom in terms of a protector’s turn-on point is the first step in protecting one’s investment. The specified clamping voltage of SPDs should be as close to the value of the AC input power as possible. While it’s logical to think that a protective device should derail overvoltages as soon as the input exceeds specified levels, actually, it turns out not to be the case. Real life power, as supplied by most utility companies, routinely varies from the nominal value, and in some cases, exceeds 15% variation above and below. How does a well-designed SPD compensate for real world power variations? Recognizing the need for G N 3 N 1 N N 3 1 G N N 2 3 N G 2 Tie Wrap or Tape Tie Wrap or Tape Tie Wrap or Tape Axial power cabling approach forces magnetic field cancellation within this cable. This results in a correspondingly low inductive voltage drop along the cable length. adequate headroom is a good place to start. Headroom is the difference in voltage between the peak of the sine wave and a higher threshold voltage level where the protective components in SPDs—metal oxide varistors (MOVs)—start to turn on. If the headroom spread is too small, the MOV may conduct more frequently, resulting in a shorter MOV life. Under unremarkable conditions, MOV life http://www.csemag.com | 23 http://www.csemag.com
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